1. Field of the Invention
The invention relates to a device for reducing drift in an electrical measuring instrument being exposed to a flowing medium, especially in an air flow rate meter for internal combustion engines, wherein the measuring instrument has a temperature-dependent measuring resistor located between two terminals on a substrate that is retained in a mount.
When electrical measuring instruments are exposed to a flowing medium, such as air, for instance in order to measure the temperature thereof, deposits in the form of a film of dirt develop on these measuring instruments over time and can lead to a shift (drift) in the characteristic curve of the measuring instrument, if the measuring instrument, for instance, includes a resistor film disposed on a substrate. Particularly with high-impedance components, such a deposit creates parallel resistors, because especially in internal combustion engines, most of the components of the dirt in the air are electrically conductive soot.
For this reason, various provisions have been employed in an attempt to prevent deposits in the form of a dirt film on such a measuring instrument.
As a representative example of such provisions, a body facing into the flow direction has been disposed upstream of an air flow rate meter disposed in the measuring channel. The body places the measurement range of the air flow rate meter in a shadow, as it were, in order to prevent the flow from directly striking it. A deposition of dirt can be at least partially avoided in that way.
As mentioned above, dirt deposits lead to parallel resistors. In turn, that results in particular in a change in the resistance between the anode and the cathode of the measuring instrument. That effect is reinforced by the moisture in the air. Over relatively long operation or in the event of major soiling, as in the case in Diesel engines, the dirt combined with the moisture from the air causes electrolysis to occur between the terminal contacts (anode, cathode). Particularly in the case of platinum resistors at the terminal contacts, it causes them to separate from the substrate, and therefore the functional reliability of the measuring instrument is no longer assured.